Resin molded articles and method of manufacturing the same

ABSTRACT

In a method of manufacturing a resin molded article, resin including a foaming agent is injected into a cavity defined in a mold assembly. Then, a movable mold of the mold assembly is moved to partly expand the cavity so that the foaming is promoted at the part where the cavity is expanded. Therefore, a high foamed portion having voids therein and a low foamed portion having an expansion ratio less than that of the high foamed portion are integrally formed in the resin molded article.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/437,602 filed May 14, 2003 which is based on Japanese PatentApplication No. 2002-141065 filed on May 16, 2002, the disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to resin molded articles and a method ofmanufacturing the same. More particularly, the present invention relatesto resin molded articles defining voids therein and a method ofmanufacturing the same.

BACKGROUND OF THE INVENTION

As an example of a resin molded article, there is an air conditioningcase of a vehicular air conditioning unit shown in FIG. 8. As shown inFIG. 8, the air conditioning case 101 includes a top case 111, a bottomcase 112, and thermal protection sheets 120. An evaporator 102 isprovided in the case 101. The top and bottom cases 111, 112 are moldedarticles without having foam structure therein. The thermal protectionsheets 120 are molded articles having foam structure.

Generally, when heat on an outer periphery of the case is absorbed bythe evaporator and the condensed water, dew drops are generated on theouter faces of the case. In the case 101, the thermal protection sheets120 are arranged at portions where the case 101 makes contact with theevaporator 102 and where condensed water from the evaporator 102 flowsto restrict generation of the dew drops and further to restrict the dewdrops from falling in a passenger compartment.

In manufacturing the case 101, the top case 111, the bottom case 112 andthe thermal protection sheet 120 are separately molded, and then thethermal protection sheets 120 are fixed in the top case 111 and thebottom case 112. Therefore, a manufacturing process is complicated.

As another example of the resin molded article, JP-B2-2625576 (U.S. Pat.No. 4,473,665) discloses a method of producing an article having foamstructure therein. According to the method, voids are uniformly formedin the article, even at a portion where the foam structure is notrequired. Therefore, if an air conditioning case is produced by themethod, the size of the case is likely to increase to ensure strength ofportions where the foam structure is not required.

SUMMARY OF THE INVENTION

The present invention is made in view of the foregoing disadvantages andit is an object of the present invention to provide resin moldedarticles and a method of manufacturing the same by a simple process andwithout increasing in size of the articles.

According to a resin molded article of the present invention, a firstwall defining voids therein and a second wall having an expansion ratioless than that of the first wall are integrally molded.

Accordingly, it is unnecessary to separately mold articles havingdifferent expansion ratio and to assemble them together. Therefore, itmakes a manufacturing process simple. Since the first wall having theexpansion ratio higher than that of the second wall is partly formed inthe molded article, the molded article is compact. Also, thermalconductivity difference can be created in the single molded article.

According to a method of manufacturing a resin molded article of thepresent invention, resin including a foaming agent is injected into acavity defined in a mold assembly first and then foamed in the cavity.The foaming of the resin is partly promoted by a foam-promoting deviceprovided in the mold assembly, thereby creating expansion ratiodifference in the molded article.

According to the method, the molding article having expansion ratiodifference therein can be produced.

Preferably, the foaming is partly promoted by partly expanding thecavity by a movable core. Alternatively, a mold wall defining the cavitycan be partly vibrated by a vibrator. Further, temperature differencecan be created in the mold wall by a temperature difference generatingmeans.

In place of partly promoting the foaming by the foam-promoting device,the cavity can be partly narrowed by a movable core so that theexpansion ratio of the molded article can be partly reduced at thenarrowed part.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description made withreference to the accompanying drawings, in which like parts aredesignated by like reference numbers and in which:

FIG. 1 is a schematic cross-sectional view of an air conditioning caseproduced by resin molding according to the first embodiment of thepresent invention;

FIG. 2 is a schematic cross-sectional view of a mold assembly accordingto the first embodiment of the present invention;

FIG. 3A is a schematic cross-sectional view of the mold assembly forexplaining an injecting step of a method of manufacturing a resin moldedarticle according to the first embodiment of the present invention;

FIG. 3B is a schematic cross-sectional view of the mold assembly forexplaining a foaming step of the method according to the firstembodiment of the present invention;

FIG. 4 is a schematic cross-sectional view of a mold assembly forexplaining a manufacturing method according to the second embodiment ofthe present invention;

FIG. 5 is a schematic cross-sectional view of a mold assembly forexplaining a manufacturing method according to the third embodiment ofthe present invention;

FIG. 6A is a schematic cross-sectional view of a mold assembly forexplaining an injecting step of a manufacturing method according to thefourth embodiment of the present invention;

FIG. 6B is a schematic cross-sectional view of the mold assembly forexplaining a foaming step according to the fourth embodiment of thepresent invention;

FIG. 7 is a schematic cross-sectional view of a air conditioning caseproduced by resin molding according to further another embodiment of thepresent invention; and

FIG. 8 is a schematic cross-sectional view of an air conditioning caseproduced by resin molding of a related art.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described hereinafter withreference to the drawings.

First Embodiment

A resin molded article of the embodiment forms an air conditioning case1, as shown in FIG. 1. FIG. 1 shows a schematic cross-sectional view ofthe case 1 in which an evaporator 2 is arranged. The case 1 includes afirst case (top case) 11 and a second case (bottom case) 12. The firstcase 11 has a contact portion 11 a that makes contact with theevaporator 2 on its inside surface (inside top surface in FIG. 1). Also,the second case 12 has contact portions 12 a that make contact with theevaporator 2 on ends of its inside surface (inside bottom surface inFIG. 1).

The second case 12 has a drain port 14 in its bottom middle. Condensedwater generated on surfaces of the evaporator 2 is discharged from thecase 1 through the drain port 14. Walls 13 between the contact portions12 a and the drain port 14 are sloped so that the condensed water flowstoward the drain port 14.

The first case 11 and the second case 12 are made of polypropylene andhave foam structure (voids) therein. Each of the first and the secondcases 11, 12 has a high foamed portion 21 and a low foamed portion 22that have different expansion ratios. In the first case 11, the topwall, which forms the contact portion 11 a inside, is formed of the highfoamed portion 21. Also, wall of the first case 11 other than the topwall are formed of the low foamed portion 22. In the second case 12, thebottom wall, which forms the contact portions 12 a, the sloped walls 13and the drain port 14, is formed of the high foamed portion 12. Also,the walls other than the bottom wall are formed of the low foamedportion 22.

In this embodiment, an expansion ratio of the high foamed portion 21 toa part without having foam structure (non-foamed portion) isapproximately 4.0. Also, an expansion ratio of the low foamed portion 22to the non-foamed portion is approximately in a range 1.1 to 1.2.Therefore, the first case 11 and the second case 12 are provided by theresin molded products that have thermal insulation at the high foamedportion 21 and reduces weight at low foamed portion 22 while maintainingstrength.

Next, a method of manufacturing the case 1 will be described. Becausethe first case 11 and the second case 12 are produced in a similarmanner, the method will be described mainly in a case of the second case12.

As shown in FIG. 2, a mold assembly 50 for injection molding includes afixed mold 51 and a movable mold 52. A mold cavity 54 is formed betweenthe fixed mold 51 and the movable core 52 when the fixed mold 51 isjoined with the movable mold 52.

The movable mold 52 is provided with a pin 52 at its inner middleportion for forming the drain port 14. Also, the movable mold 52includes a movable core 53 that is movably supported in a top and bottomdirection in FIG. 2. The movable core 53 is connected to an actuator 55that is provided outside of the movable mold 52 as a movablecore-driving means. The movable core 53 is moved in the top and bottomdirection by operation of the actuator 55, thereby changing a volume ofthe cavity 54.

In the embodiment, a hydraulic actuator having a hydraulic cylinder isused as the actuator 55. Further, as an example of the actuator 55, apneumatic actuator and a motor-driven actuator can be used.

In molding the case 12, first, the fixed mold 51 and the movable mold 52are joined as shown in FIG. 3A. Then, a molten resin 20 including afoaming agent is injected from a gate (not shown) into the mold cavity54. In the embodiment, polypropylene resin is used as the resin 20.Nitrogen molecules in a supercritical liquid state are contained in theresin 20 as the foaming agent.

A supercritical liquid is defined as a material maintained under apressure over a critical pressure and a temperature over a criticaltemperature. Under that condition, the material is in a supercriticalliquid state. N₂ (nitrogen) used as the foaming agent is in asupercritical state when maintained under a pressure over 3.4 MPa and atemperature over −147 degrees Celsius.

The supercritical liquid is characterized by acting as gas and liquid.Therefore, supercritical liquid nitrogen is readily diffused and mixedin the polypropylene resin. In the embodiment, supercritical liquidnitrogen of 0.6 percent by weight is included in the resin 20.

After the resin 20 is injected into the mold cavity 54, as shown in FIG.3B, the movable core 53 is moved in a direction expanding the volume ofthe cavity 54 (downward in FIG. 3), by operation of the actuator 55. Inthe embodiment, a mold temperature is 15 degrees Celsius. The movablecore 53 is moved approximately two seconds later a completion of theresin injection.

The supercritical liquid nitrogen in the resin 20 gradually vaporizesimmediately after injected in the cavity 54 and forms foam nuclei. Asthe vaporization of the supercritical liquid nitrogen continues, foamnuclei grow and forms voids, thereby forming foam structure in the resin20. At a part where the cavity 54 is expanded by movement of the movablecore 53, forming of the foam nuclei is accelerated or promoted, therebyincreasing the expansion ratio.

In this way, the second case 12 in which the high foamed portion 21 andthe low foamed portion 22 are integrally molded is produced. Since theexpansion ratio of the high foamed portion 21 is approximately 4.0,voids may be communicated therein. However, thin resin layers (skinlayers) are formed on the surfaces of the molded product. Therefore, thewall of the second case 12 does not have communication structureallowing communication between the inside surface and the outsidesurface.

In the manufacturing method, the step shown in FIG. 3A is referred to asan injecting step and the step shown in FIG. 3B is referred to as afoaming step.

According to the above method, it is possible to create expansion ratiosdifference in a single article. Therefore, it is unnecessary toseparately mold articles having different expansion ratios and toassemble then together to make the first case 11 and the second case 12.Accordingly, a manufacturing process is simple. Further, the high foamedportions 21 are formed partly in the first case 11 and the second case12, respectively. Therefore, the resin molded article is compact.

The high foamed portion 21 has a thermal conductivity of 0.5 W/(m·K)even when the evaporator 2 is arranged in the case 1. Therefore, dewdrops less likely to generate on outer faces of the case 1.

Especially, in the case 1, since the condensed water flows through thedrain port 14, the drain port 14 is cooled more than the other portionin the case 1. Because the drain port 14 is formed of the high foamedportion 21, it is effective to restrict generation of dew drops.

Second Embodiment

In the second embodiment, the high foamed portion 22 is formed in amanner different from that of the first embodiment.

As shown in FIG. 4, the movable mold 52 includes a body portion 61 andan insert portion 62. A vibrator 64 is provided under the insert portion62 as a vibrating device. The insert portion 62 vibrates by operation ofthe vibrator 64. Gaps 63 are defined partly between the body portion 61and the insert portion 62 to restrict the body portion. 61 fromvibrating due to vibrations of the insert portion 62. As examples of thevibrator 64, an ultrasonic vibrator and air vibrator can be used.

When the second case 12 is molded, first, the resin 20 including thefoaming agent is injected into the cavity 54 in a manner similar to thefirst embodiment. Then, the vibrator 64 is turned on to vibrate theinsert portion 62. With this, a surface 62 a of the insert portion 62,which defines the cavity 64, vibrates. Therefore, foam nucleation of theresin 20 is promoted at the vibrated portion. As a result, the expansionratio of the vibrated portion is higher than the other portion.

In this way, the high foamed portion 21 and the low foamed portion 22are formed integrally in the second case 12. Here, the step of vibratingthe insert portion 62 corresponds to the foaming step.

Similar to the first embodiment, it is unnecessary to mold articleshaving different expansion ratio separately and to assemble them toconstruct the first case 11 and the second case 12. Therefore, themanufacturing method is simple. Also, the high foamed portions 21 areformed partly in the first case 11 and the second case 12. Therefore, itis possible to make the resin molded article compact.

Third Embodiment

In the third embodiment, the high foamed portion 21 is formed in amanner different from that of the first embodiment.

As shown in FIG. 5, the movable mold 52 includes the body portion 61 andthe insert portion 62. An electric heater 74 is embedded in the insertportion 62 from the bottom. The electric heater 74 is a heating deviceand heats the insert portion 62 when it is electrically conducted. Thegaps 63 are formed partly between the body portion 61 and the insertportion 62 to prevent the body portion 61 from receiving heat from theinsert portion 62. Instead of the gaps 63, a heat insulation materialcan be provided between the body portion 61 and the insert portion 62.

When the insert portion 62 is heated by operation of the electric heater74, the surface 62 a of the insert portion 62, which defines the cavity54, is heated and the temperature of the surface 62 a is higher than theother surrounding walls. The electric heater 74 is a temperaturedifference-generating device to create temperature differences in wallsdefining the cavity 54.

In molding the second case 12, first, the resin 20 including the foamingagent is injected in the mold cavity 54 in a manner similar to the firstembodiment. At this time, the insert portion 62 has been heated by theelectric heater 74. In the embodiment, the body portion 61 is maintainedat 15 degrees Celsius and the insert portion 62 is heated at 80 degreesCelsius. Thus, the nucleation is promoted at a part adjacent to thesurface 62 a that is approximately 80 degrees Celsius. As a result, theexpansion ratio of the part adjacent to the surface 62 a is higher thanthat of the other part.

In this way, the second case 12 in which the high foamed portion 21 andthe low foamed portion 22 are integrally formed can be produced. FIG. 5illustrates the injecting step and the foaming step of the method.

Also in this embodiment, it is unnecessary to separately mold thearticles having different expansion ratios and to assemble them toconstruct the first case 11 and the second case 12. Therefore, themanufacturing process is simple. Also, the high foamed portions 21 areformed partly in the first case 11 and the second case 12. Therefore,the resin molded article is compact.

Fourth Embodiment

In the fourth embodiment, the high foamed portion 21 and the low foamedportion 22 are formed in a manner different from that of the firstembodiment.

As shown in FIG. 6A, the movable mold 52 includes a plurality of movablecores 83. The movable cores 83 are supported slidably in the left andright direction in FIG. 6A. The movable cores 83 are respectivelyconnected to actuators 85 that are provided on the outside of themovable mold 52 as a movable core driving means. The walls of themovable cores 83 defining the cavity 54 are moved by operation of theactuators 85 so that the volume of the cavity 54 is partly changed.

In molding the second case 12, first, the resin 20 including the foamingagent is injected into the cavity 54 in a manner similar to the firstembodiment, as shown in FIG. 6A. Then, as shown in FIG. 6B, the movablecores 83 are moved by the actuators 85 inwardly, that is, in thedirection that the volume of the cavity 54 is reduced. In theembodiment, the resin 20 is injected into the cavity 54 maintained atthe mold temperature of 80 degrees Celsius. The movable cores 83 aremoved approximately two seconds later the completion of the resininjection.

At the part where the volume of the cavity 54 is reduced, the nucleationin the resin 20 is limited and the foams grown therein are compressed.Therefore, the expansion ratio of the compressed part is smaller thanthe other parts.

In this way, the second case 12 in which the high foamed portion 21 andthe low foamed portion 22 are integrally formed can be produced. In theembodiment, the step shown in FIG. 6A corresponds to the injecting stepand the step shown in FIG. 6B corresponds to the foaming step.

Also in this embodiment, it is unnecessary to separately mold partshaving different expansion ratios and to assemble the parts to constructthe first case 11 and the second case 12. Therefore, the manufacturingprocess is simple. Also, the high foamed portions 21 are formed partlyin the first case 11 and the second case 12, respectively. Therefore, itis possible to make the resin molded article compact.

As another modification, another supercritical liquid can be used as thefoaming agent, in place of the supercritical liquid nitrogen. Forexample, the foaming agent can be supercritical liquid carbon dioxide.Further, the bowing agent is not limited to the supercritical liquid.For example, it can be a physical foaming agent that foams byevaporation. Also, it can be a chemical foaming agent that foams bycracked gas.

In the case 1 described in the above embodiments, the high foamedportions 21 and the low foamed portion 22 are integrally molded.However, the case 1 can be a molded article in which a foamed portion 21a defining voids therein and a non-foamed portion 22 a without definingvoids therein are integrally molded, as shown in FIG. 7. The non-foamedportion 22 a is formed by avoiding foam nucleation in a method similarto that of the above-described embodiments.

In the third embodiment, the electric heater 74 is used as the heatingdevice. However, another type of heater, such as a glow plug, can beused. Also, the heater 74 is used as a device to create temperaturedifference in the mold 50. However, the temperature difference can becreated by other methods. For example, passages through which coolant(e.g. cold water) and heating medium (e.g. hot water) flow can be formedin the mold. By this, the temperature differences can be created on thewalls of the mold defining the cavity.

The resin 20 is not limited to the polypropylene resin. Another resinmaterial can be used to the present invention.

In the above-described embodiment, the movable core 53, vibrator 64,electric heater 74 and the movable cores 83, are arranged on a side ofthe movable mold 52. However, the arrangements are not limited to theabove. Those can be arranged other positions, for example, in the fixedcore 51.

Further, the devices, such as the movable the core 53, the actuator 55,the insert portion 62, the vibrator 64 and the electric heater 74, areused as foam-promoting devices to create expansion ratio differences inthe singe molded article. Those devices can be used in variablecombinations. In the above embodiments, the method is explained in acase of molding the air conditioning case as an example. The product isnot limited to the air conditioning case. The method of the presentinvention can be used to produce molded articles for other purposes.

The present invention should not be limited to the disclosedembodiments, but may be implemented in other ways without departing fromthe spirit of the invention.

1. A method of manufacturing a resin molded article comprising:injecting resin including a foaming agent into a cavity defined in amold assembly; and foaming the injected resin in the mold assembly,wherein the foaming of the resin is partly promoted in the cavity by afoam-promoting device provided in the mold assembly, thereby creatingexpansion ratio difference in the molded article.
 2. The methodaccording to claim 1, wherein the foam-promoting device includes amovable core, wherein the movable core moves to partly expand the cavityafter the resin injection, thereby increasing the expansion ratio of theresin at the part where the cavity is expanded.
 3. The method accordingto claim 1, wherein the foam-promoting device includes a vibratordisposed to partly vibrate a mold wall defining the cavity, wherein thevibrator operates during the foaming, thereby increasing the expansionratio of the resin at a vibrated part.
 4. The method according to claim1, wherein the foam-promoting device includes a temperature differencegenerating means disposed to create temperature difference in a moldwall defining the cavity during the foaming, thereby increasing theexpansion ratio at a portion where temperature is higher than the otherin the mold wall.
 5. The method according to claim 4, wherein thetemperature difference generating means includes a heater disposed topartly heat the mold wall.
 6. The method according to claim 1, whereinthe foaming agent includes supercritical liquid nitrogen.
 7. The methodaccording to claim 1, wherein the foaming agent includes supercriticalliquid carbon dioxide.
 8. A method of manufacturing a resin moldedarticle comprising: injecting resin including a foaming agent into acavity defined in a mold assembly; and foaming the injected resin in themold assembly, wherein during the foaming the cavity is partly narrowedby a movable core provided in the mold assembly, thereby reducing anexpansion ratio of the molded article at a part where the cavity isnarrowed.
 9. The method according to claim 8, wherein the foaming agentincludes supercritical liquid nitrogen.
 10. The method according toclaim 8, wherein the foaming agent includes supercritical liquid carbondioxide.
 11. A method of manufacturing a resin molded articlecomprising: preparing injection material including resin and a foamingagent; defining a cavity in a mold; injecting the prepared injectionmaterial into the cavity so that the cavity is filled with the injectionmaterial; foaming the injection material injected in the cavity by thefoaming agent; while the foaming process, differentiating a foamingratio of the injection material filled in at least two locations in thecavity; and opening the mold to eject a molded article which hasintegrally molded at least two portions having different foaming ratio.12. The method according to claim 11, wherein the mold defines a thickerportion and a thinner portion in the cavity, and the differentiatingstep provides a higher foaming ratio in the injection material filled inthe thicker portion.
 13. The method according to claim 12, wherein thedifferentiating step is performed by adjusting a thickness of the cavityafter the cavity is filled with the injection material.
 14. The methodaccording to claim 12, wherein the differentiating step is performed bywidening the cavity to define the thicker portion during the foamingstep, widening the cavity being capable of increasing the foaming ratioof the injection material filled in the thicker portion.
 15. The methodaccording to claim 14, wherein the widening the cavity is performedafter the injecting step and is performed during the foaming step. 16.The method according to claim 12, wherein the differentiating stepincluding the step of vibrating a portion of the mold defining thethicker portion to increase the foaming ratio of the injection materialfilled in the thicker portion.
 17. The method according to claim 12,wherein the differentiating step differentiates temperature of portionsof the mold such that a portion defining the thicker portion is highertemperature than a portion defining the thinner portion, the highertemperature being capable of increasing the foaming ratio of theinjection material filled in the thicker portion.
 18. The methodaccording to claim 12, wherein the differentiating step including thestep of heating the portion of the mold defining the thicker portion toincrease the foaming ratio of the injection material filled in thethicker portion.
 19. The method according to claim 12, wherein thedifferentiating step is performed by narrowing the cavity to define thethinner portion during the foaming step, the narrowing the cavity beingcapable of decreasing the foaming ratio of the injection material filledin the thinner portion.
 20. The method according to claim 19, whereinthe narrowing the cavity is performed after the injecting step and isperformed during the foaming step.